The correct option is C Red light
Important plant responses regulated by the phytochrome system include photoperiodic induction of flowering, chloroplast development, leaf senescence and leaf abscission. Phytochrome occurs in two forms: Pr (red light) and Pfr (far red light) and both are interconvertable. It is considered that during day time Pfr form accumulates in plants which is inhibitory to flowering in SDP. During night Pr form accumulates in plants which is stimulatory to flowering.